OPHIR BeamSquared, User Manual

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Waist Width 6.1.2.2
Equation 8 – Waist Width
𝑊
01
=
4𝜆𝑀
2
𝜋𝛩
1
Beam Width Basis
As discussed in section 5.4.2, D4σ is the preferred method for computing a beam width. For beams with M
2
values approaching 1, the Knife Edge 10/90 method is also very accurate.
The Beam Width Basis selection determines the technique used to compute the beam width results. Three
of the methods (D4σ, Knife Edge 10/90, and EPSA) are computed based on industry standard definitions. The
remaining Knife Edge choice can be used under rare circumstances where the other choices may not be
optimum.
The D4σ method directly computes second moment beam widths in the X and Y beam axial directions. D4σ is
most sensitive to noise; however laser mode content does not influence the computations, a significant
advantage.
The Knife Edge 10/90 method employs a fixed 10% and 90% of energy as the moving edge clip points. The
correction multiplier is fixed at 1.561. These settings yield highly accurate second moment equivalent beam
widths for beams that are predominantly TEM
00
, as well as for many other mixed mode combinations. There
are a few modes for which this method will not prove as accurate such as Top Hat shaped beams.
If laser mode content is not suitable for measurement using either of the previous methods, use the Knife
Edge Prog method and enter custom clip levels and a multiplier. Suggested multiplier values are:
 For a circular Top Hat, enter a Multiplier of 1.455

For a square Top Hat, enter a Multiplier of 1.444
ISO presets clip levels of 16% and 84% with a multiplier of 2.0. This works well with beams that are mostly
TEM
00
.
In general, the two Clip level settings will add up to 100%. Avoid clip levels <10% as this setting
begins to approach the camera’s noise floor.
EPSA Prog is the Encircled Power Smallest Aperture method, also commonly called the encircled power or
power in a bucket method. Enter the percent of the total power for the reported beam diameter to encircle.
Because the aperture is always circular, the X and Y results will always report the same values. 86.5% is the
default setting and represents the second moment setting for a TEM
00
Gaussian beam.